1. Field of the Invention
The present invention generally relates to a plasma etching machine capable of cleaning organic and inorganic substances from a variety of materials and, more particularly, to a plasma etching machine comprising a conventional microwave oven which has been modified to perform this cleaning function.
2. Description of the Prior Art
Plasma assisted etching and cleaning techniques are recognized to have many advantages over chemical etching and cleaning. Plasma cleaning consumes very small quantities of reactants as opposed to the large quantities used in chemical techniques. No significant effluent is produced during plasma cleaning, therefore plasma cleaning does not have a waste disposal problem found with chemical etching. Substrate temperatures are relatively low during processing thereby eliminating damage to the substrate caused by heat exposure. Plasma etching is used in very large scale integrated (VLSI) circuit fabrication because of its superior dimensional control whereby an accurate replication of a photoresist pattern may be etched into an underlying film.
Barbara A. Heath and Lee Kammerdiner have outlined four basic types of equipment used in plasma assisted etching in VLSI Handbook, edited by Norman G. Einspruch, published by Academic Press, Inc. in 1985, pages 490 and 491. FIG. 1 shows a barrel etcher where the plasma serves to produce a reactive etchant species. The plasma is often isolated from the wafers by an etch tunnel, leaving the reactant species to reach the wafers by diffusion. The design of the reactor usually results in an isotropic etch profile. Barrel etchers are commonly used for photoresist stripping. FIG. 2 shows a planar plasma (diode) reactor wherein the wafers are loaded onto a grounded electrode. FIG. 3 shows a reactive ion etcher (RIE) wherein the wafers are loaded on a powered electrode. Low pressure operation and the relative size of the electrode to the rest of the chamber cause a direct current (dc) bias to appear on the powered electrode in the RIE system. Ion assisted anisotropic etching results from significant positive ion bombardment of the electrode surface. FIG. 4 shows an ion milling apparatus wherein an ion beam is produced and directed onto the sample. The ion producing plasma is isolated from the sample to be etched.
Plasma is a high density gas of charged particles. Microwaves have been used for several years to produce plasmas. A. Ribner and O. A. Weinreich described the use of a microwave generator to make a plasma for chemical synthesis in "Optical and Electrical Properties of SiC Films Prepared in a Microwave Discharge", J. Electrochem. Soc., vol. 115, page 1090, 1968. Microwave plasma etching has been disclosed in the following patents: U.S. Pat. No. 4,138,306 to Niwa, U.S. Pat. No. 4,357,203 to Zelez, U.S. Pat. No. 4,430,138 to Suzuki et al, U.S. Pat. No. 4,462,863 to Nishimatsu et al, U.S. Pat. No. 4,581,100 to Hazakis et al, U.S. Pat. No. 4,599,100 to Ninomiya, and U.S. Pat. No. 4,609,428 to Fujimura. The machines disclosed in the above listed U.S. Patents are expensive to manufacture and, therefore, a need exists for a less expensive microwave plasma etching machine.
Microwave ovens have become a ubiquitous commodity in today's modern household. Microwave ovens are generally used to cook food and the ability to cook food an order of magnitude more rapidly than a convection oven has been the driving force behind their popularity. U.S. Pat. No. 3,887,392 to Tang discloses a method for using microwave energy to cook foods and to clean semiconductors. When cooking food, a sheet impregnated with oil and seasonings is wrapped around an article of food. Microwave heat allows the seasonings to drip out of the sheet and properly flavor the food. When cleaning semiconductors, a solvent impregnated sheet is spread over the wafers. The solvent mixes with the dirt and oils on the wafers and microwave energy vaporizes the solvent, thereby carrying off the contaminants on the wafers. This method employs a chemical cleaning process which requires dissolving the contaminants in a solvent such as alcohol. The user is required to handle a sheet impregnated with solvent. Organic solvents are known to aid carcinogens in penetrating the human body. Therefore, the cleaning technique contemplated by Tang is hazardous. Moreover, Tang does not contemplate stripping photoresist material.
Early microwave oven designs tended not to cook food evenly such that some parts of a food article would be burned while other parts would be raw after a cooking cycle. Two methods for cooking food uniformly have been developed. One method includes a rotating platform inside the microwave oven. The other method involves a rotating antenna. The principle behind both concepts is to expose all parts of the food article to an equal amount of microwaves.